storm water detention tanks

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tweake

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#303653 25-Feb-2023 15:41

anyone done this at all?

just looking at storm water detention tanks due to the rain which was to much water for the pipes to carry. a previous owner had tanks here, which have long since been removed. so i was thinking about reinstating that to handle excessive rain especially as these big rain events seam to be getting more common every year.

catch is what size would i need ??

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#3041833 25-Feb-2023 15:45

Multiple expected rainfall by capture area. You will get 1L of water per square meter for each mm of rain.

k1w1k1d

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#3041841 25-Feb-2023 16:29

Before you go too far, I would suggest that you look at the price of water tanks.

neb

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#3041848 25-Feb-2023 16:52

SirHumphreyAppleby:
Multiple expected rainfall by capture area. You will get 1L of water per square meter for each mm of rain.

And if you're trying to catch something at the level of what we've had recently you're probably not going to be able to do it unless you put in fairly sizeable concrete cisterns, and you'd have to carefully manage it so the preliminary soak doesn't fill the storage volume before the real downpour that you want to deal with begins. Have a look at things like rain gardens as a way of slowing down water volumes...

tweake

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#3041904 25-Feb-2023 17:46

all it really has to catch is what ever rain is more than what existing pipes can handle. normal rain before hand won't get caught because it will drain away as fast as it comes in. its still the stock 4" pipe out.

i suspect the choke point is the house/garage underground pipe to the town storm water pipe. its made for normal heavy rain, not this monster rain we seam to get now. even at work we added 6" pipe and its like we did nothing.

neb

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#3041911 25-Feb-2023 18:30

Ah, in that case and if it's purely roof catch go with @SirHumphreyAppleby's suggestion. Even then though with a typical roof area of 150m2 and 100mm rain, a few days ago in Orkland but only a third of 31st Jan, that's 15,000L to catch and that's a big tank.

wally22

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#3041915 25-Feb-2023 18:58

The underlying principle of detention tanks in NZ does not really take into account the recent rainfalls. I interpreted the concept as a way of limiting capital outlay on infrastructure by the councils.

Benoire

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#3041962 25-Feb-2023 19:03

wally22:

The underlying principle of detention tanks in NZ does not really take into account the recent rainfalls. I interpreted the concept as a way of limiting capital outlay on infrastructure by the councils.

The concept is more about reducing the initial peak flow and spread it over a longer period to avoid overwhelming the piped network which only has a 10% AEP service level (i.e. 1 in 10 year). Beyond this, its overland flow with paths getting more pronounced and deeper as the rainfall continues.

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tweake

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#3041964 25-Feb-2023 19:14

neb: Ah, in that case and if it's purely roof catch go with @SirHumphreyAppleby's suggestion. Even then though with a typical roof area of 150m2 and 100mm rain, a few days ago in Orkland but only a third of 31st Jan, that's 15,000L to catch and that's a big tank.

but over what time period and how much is going out the outlet. you might have 15,000 litres coming into the tank but 14,000 litres is going out at the same time. your not trying to save the water, just supply a bit of space to handle those short bursts of insanely heavy rain.

neb

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#3041966 25-Feb-2023 19:23

Yeah, it depends where the chokepoint is, if it's a 100mm pipe feeding a 150mm main that's also fed by several other 100mm pipes from neighbouring properties then the tank may need to retain the entire rain load until it's possible to put more water down the outflow.

More data needed :-).

Benoire

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#3041967 25-Feb-2023 19:54

Look at smaf1/2 area requirements from the AC storm water code of practice along with the Auckland unitary plan, these will give a good idea of how much capacity you might need to actually do anything helpful.

neb

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#3041974 25-Feb-2023 20:20

Benoire: AC storm water code of practice along with the Auckland unitary plan

How realistic is that though? In other words if you follow that you can claim compliance with requirements but will it handle what we're seeing now rather than what the plan was written for?

Benoire

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#3041975 25-Feb-2023 20:28

No, TP108 (rainfall patterns for Auckland) has not been updated to reflect the shifting nature of the events, but you can generally pick up the amount from Metservice etc. and then use the calcs in these rules to see how much capacity you will need... in any avent, for my 90m2 house with 130m2 roof area a 30kL tank provided enough capacity to take the event But the subsequent days would have overflowed.

Be mindful, however, that overland flows could add to your own land area volume and reduce the ability for any tank to act as you think.

My goal for my next house is to plumb in the rainwater as the primary source for my house and then use the mains when it runs low; it will be cleaner and less metals compared to the current watercare supply and generally a standard rain event in winter in Titirangi will supply ~10 days worth of water for me (nearly 5000L) with a historic summer level in feb providing about 12 days worth of fresh water.

tweake

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#3041982 25-Feb-2023 21:07

neb: Yeah, it depends where the chokepoint is, if it's a 100mm pipe feeding a 150mm main that's also fed by several other 100mm pipes from neighbouring properties then the tank may need to retain the entire rain load until it's possible to put more water down the outflow. More data needed :-).

for eg, the 50sqm garage looks like it has 100mm pipe but it runs out to the curb, so no effect from neighbours. the pipes where full up to the gutter and the gutter was overflowing badly. clearly just to much load for the pipes.

the grate on the road wasn't overflowing so i think the storm drain was coping ok, but the house was also over flowing. not just out the overflow, but over the gutters as well. each gutter goes into 100mm but not sure what size they feed into. it may have a 150mm to the storm pipes.

the problem of course is that overflow all ends up on my footpath and flows down to the garage.

if i put a tank on the back of the house, i can plumb an overflow that goes down the back section which goes into farmland/creek.

tweake

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#3041984 25-Feb-2023 21:12

Benoire: Look at smaf1/2 area requirements from the AC storm water code of practice along with the Auckland unitary plan, these will give a good idea of how much capacity you might need to actually do anything helpful.

i'll look at that but they will be doing it for a different reason. they would be wanting to hold a large amount and put less flow into the storm water than normal. where as i'm putting normal flow into storm water. just want to hold water if that gets pushed past max.

MyFriendAutism

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#3041989 25-Feb-2023 21:58

tweake:

anyone done this at all?

just looking at storm water detention tanks due to the rain which was to much water for the pipes to carry. a previous owner had tanks here, which have long since been removed. so i was thinking about reinstating that to handle excessive rain especially as these big rain events seam to be getting more common every year.

catch is what size would i need ??

Mine is more than 5000lts(1m wide x *** high) with a pump @ the bottom. It takes surface, spouting, backyard drainage water & run off from neighbouring properties. With heavy rain it takes ~30mins to fill if I turn the pump off(I never do) & will keep filling 2-3 days after. Cyclone Gabby was an exception. City storm water failed so place flooded but once city storm water was operational I had 7 days solid refill.

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